[llvm] [VPlan] Add unit test for createAndOptimizeReplicateRegions. NFC (PR #164652)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Could we avoid this by updating the IR tests to use explicitly predicated blocks, instead of relying on tail-folding?
> 
> The C++ unit tests are a bit more cumbersome to update, so if we could avoid that it would be great

I originally tried this but there's a few test cases in first-order-recurrence-sink-replicate-region.ll like `@sink_replicate_region_1` that can't be converted to explicitly predicated blocks because the FOR can't be predicated. But for the tests that are affected by #160449 we can explicitly predicate them, if you're ok with only some of them being updated.

https://github.com/llvm/llvm-project/pull/164652